The present invention relates in general to optical cables, and more particularly to a cable having buffer tubes in which optical fibers are loosely provided. The optical (fiber) cables are used, for example, in telecommunications to transmit voice, data, video and multimedia information.
Many factors are considered when designing cables, including low costs and compact size. A compact cable design is important in order to attain a high efficiency (i.e., a high fiber count in a small cable volume). Another consideration is the cable""s performance during temperature variations in the environment in which the cable is installed. Temperature variations cause the cable to expand and contract, which leads to signal attenuation. Signal attenuation is particularly problematic in central cavity cable designs.
With reference to FIG. 1, a central cavity cable 1 has an outer sheath 2 in which buffer tubes 3 are provided. Each of the buffer tubes 3 surrounds a plurality of loosely provided optical fibers 5. The outer sheath 2 and buffer tubes 3 are typically made from plastic materials. These plastic materials have a much higher coefficient of thermal expansion than the glass materials that make up the optical fibers 5. Therefore, during temperature variations, the outer sheath 2 and buffer tubes 3 tend to deform more severely than the optical fibers 5. This relative deformation difference causes the buffer tubes 3 to bend, or in an extreme case, buckle, thereby increasing signal loss.
On one hand, the buffer tube deformation is negligible because the buffer tubes 3 are thin walled and delicate relative to the optical fibers 2. In fact, for cable compactness, it is desirable to make the buffer tubes 3 with as little material as possible. Thus, the optical fibers 5 are stiff enough to withstand and counteract the relatively weak deformation forces exerted by the delicate buffer tubes 3. Further, the buffer tubes 3 have free space in which the optical fibers 5 may move. That is, the optical fibers 5 are loosely provided in the buffer tubes 3. Therefore, some buffer tube deformation may occur without having any affect on the optical fibers 5 therein.
On the other hand, the outer sheath deformation is more problematic. In particular, the outer sheath 2 is much bulkier (made from more material per unit length) than the buffer tubes 3. Therefore, the outer sheath 2 drastically deforms due to thermal fluctuations. Moreover, the outer sheath""s deformation forces are much stronger than the optical fibers 5 are capable of withstanding.
Consider, for example, a scenario in which the temperature of the environment drops from a relatively high temperature as shown in FIG. 2(A) to a relatively low temperature as shown in FIG. 2(B). FIG. 2(A) shows the cable 1 in a non-buckled state. At some point 10 along the length of the cable 1, the inner surface of the outer sheath 2 frictionally engages with the outer surface of one of the buffer tubes 3xe2x80x2 (xe2x80x9ca contacting buffer tubexe2x80x9d).
Turning to FIG. 2(B), as the temperature drops, the buffer tubes 3, 3xe2x80x2 may contract (or deform) slightly in a longitudinal direction 15. But this contraction is counteracted by the stiffness of the optical fibers 5, or altogether avoided due to the free space within the buffer tubes 3, 3xe2x80x2. However, due to its bulkiness, the outer sheath 2 contracts severely in the longitudinal direction 15. The bulkiness of the outer sheath 2 also creates substantial contraction forces. The frictional engagement at the contact point 10 effectively combines the contraction forces from the outer sheath 2 and the contacting buffer tube 3xe2x80x2. These combined contraction and contact forces overcome the stiffness of the optical fibers 5 in the contacting buffer tube 3xe2x80x2. Therefore, as the contact point 10 moves to the right (for example) a certain distance d, the contacting buffer tube 3xe2x80x2 bends, and in extreme conditions, buckles. Eventually, the inner diameter of the contacting buffer tube 3xe2x80x2 engages with and bends the optical fibers 5 therein. This phenomenon of fiber microscopic bending/buckling due to the combined contacting and frictional forces is known in the art as micro bending. Micro bending increases signal loss.
Conventionally, two techniques have been employed to overcome the micro bending resulting from thermal deformation of cable elements. The first technique involves incorporating bigger and more anchoring elements into the center of the cable""s central cavity. FIG. 2(C) illustrates one example of a centrally located anchoring element, which is referred to in the art as a central strength member 4. The buffer tubes 3 are stranded around the central strength member 4 during cable fabrication. In this way, the central strength member 4 serves to xe2x80x9canchorxe2x80x9d the buffer tubes 3. The central strength member 4 is formed from materials that are stiff, and have very small thermal deformation characteristics. Consequently, the central strength member 4 provides buckling resistance and counteracts outer sheath contractions. The second technique is to design the cable 1 with increased free space in which the buffer tubes 3 or optical fibers 5 are moveable. This increased free space enables the optical fibers 5 to readily move away from a buckled portion of the contacting buffer tube 3xe2x80x2.
Although these conventional techniques are generally thought to be acceptable, they have shortcomings in terms of design efficiency. Namely, both techniques increase cable dimensions, and therefore reduce cable efficiency (i.e., smaller fiber count per cable volume). Furthermore, the centrally located anchoring members significantly reduce the cable""s flexibility, which is particularly problematic for some applications.
It is therefore an object of this invention to provide a unique cable design having improved performance throughout temperature variations that occur in the installed environment. More specifically, the object of this invention is to effectively avoid optical fiber micro bending (and the associated signal loss) that result from thermal deformation, without reducing the fiber count per cable volume.
The invention resides in a cable having an outer sheath with a central cavity in which a plurality of buffer tubes is provided. At least one optical fiber is provided in each of the buffer tubes. The buffer tubes are coupled together to prevent slippage between the buffer tubes. The coupled together buffer tubes have an increase buckling resistance. According to one aspect of the invention, an adhesive couples together the buffer tubes. According to another aspect of the invention, the buffer tubes are fused together.
The above and other features of the invention including various and novel details of construction will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular cable embodying the invention is shown by way of illustration only and not as a limitation of the invention. The principles and features of this invention may be employed in varied and numerous embodiments without departing from the scope of the invention.